docs(surface): hide impl-detail re-exports + clarify schema_version + dogfood note

[luofang34]

Summary

Three shrink-and-document moves toward a leaner pre-1.0 public surface, no code-behavior change:

• evidence-core lib.rs: re-exports split into a curated public surface (~36 items, grouped by external use case) and a #[doc(hidden)] implementation-detail block (~38 leaf items: BoundaryViolation, BranchCoverage, FixHint, sha256, rules_json, TraceMeta, etc.). Hidden items stay reachable via their owning module path for workspace callers and contract tests; they drop off rustdoc.
• schema_versions.rs + README "Project Status": spell out that the on-disk schema_version field is informational during pre-1.0 — pinned at 0.0.1 across breaking shape changes, not a compatibility contract. Consumers should pin a cargo-evidence workspace version (read engine_crate_version in index.json) instead of pattern-matching the literal.
• CONTRIBUTING.md: new "Dogfooding doctor / hint / MCP behavior changes" section. The installed binary on $PATH is the wrong test target when iterating on user-facing output. Use cargo run --bin cargo-evidence -- evidence doctor for direct CLI work; cargo install --path crates/cargo-evidence --path crates/evidence-mcp --force before re-invoking the MCP wrapper. MCP_VERSION_SKEW catches binary-vs-binary drift but not workspace-vs-binary drift.

This is the first of two planned pre-1.0-honesty PRs. The follow-up (separate) renames the HMAC envelope away from "signature" terminology (sign_bundle → compute_bundle_mac, BUNDLE.sig → BUNDLE.mac, SYS-001 wording) so the project stops calling a shared-secret MAC a digital signature. That PR will trip the cross-time determinism gate and will use the Override-Deterministic-Baseline: protocol; this one does not.

Test plan

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets (all green; floors_equal_current_no_slack passes)
• RUSTDOCFLAGS='-D missing_docs -D rustdoc::broken_intra_doc_links' cargo doc --workspace --no-deps
• cargo evidence trace --validate → TRACE_OK
• cargo evidence floors --format=jsonl → FLOORS_OK
• cargo evidence doctor → no DOCTOR_FLOORS_SLACK, all checks [✓]
• CI green on push (cross-host + Nix flavors)

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